1. Field of the Invention
The present invention relates to an electron source which operates on the principle of field emission. The aim of the invention is to improve sources of this type, particularly when they are constructed by means of methods relating to the technology of integrated circuits or to the field of deposition of thin films on a substrate as is the case, for example, with the fabrication of MOS transistors.
2. Description of the Prior Art
Over the past few years, the techniques already employed for integrated circuits or in the field of thin films have led to substantial progress in the fabrication of field-emission electron sources. These techniques make it possible in particular to obtain structures having very small dimensions which utilize in each case a point having a very small radius of curvature. The point is made emissive under the influence of an electric field produced by means of an electrode which is brought to a positive potential with respect to the potential of the point. This structure having a point constitutes an elementary electron-emitting device which can form a microtube of the triode type, for example, or else an electron microgun, and this elementary device may be employed alone or combined with other similar devices.
The operation and the methods of construction of a field-emission electron source formed by a plurality of elementary emitter devices are known in particular from studies carried out at Stanford Research Institute by C. A. Spindt and published in various reviews including Applications of Surface Science, 2, pages 149-163 (1979) and Applications of Surface Science 16 (1983) pages 268-276, as well as the Journal of Applied Physics, vol. 47, No. 12, December 1976, pages 5248-5263.
Another document which may also be cited is French patent Application No. 2,568,394 which mentions the researches of C. A. Spindt and describes different modes of operation and construction of cathodes each formed by a plurality of micropoints which emit electrons in accordance with the principle of field emission. Each micropoint is capable of emitting an electron beam which bombards a cathodoluminescent anode forming the screen of a visual display device. Examples of utilization and fabrication of micropoints in order to constitute field-emission cathodes are also found in a French patent Application No. 80 26934 published under No. 2,472,264, and in U.S. Pat. No. 4,513,308.
FIG. 1 of the accompanying drawings illustrates schematically by way of example an elementary field-emission electron emitter device of known type. The emitter device 1 is formed on a substrate 2 which is shown partly but the dimensions of which can permit the construction of a plurality of emitter devices 1 placed side by side in a matrix arrangement, for example. The substrate 2 is of semiconductor material such as silicon, for example, but could also be a conductive layer such as aluminum, for example. In the example shown, the substrate 2 is cut so as to have a well 3 at the center of which remains a protuberance 4 of conical shape. The well 3 is centered about an axis 5 which is intended to constitute the axis of an electron beam 6. Thus, in the example illustrated, the protuberance or cone 4 is of the same material as the substrate 2, its base is integral with the bottom wall of the well 3, its summit or point 7 is oriented towards the exterior of the well 3 and located on the longitudinal axis 5. It is worthy to note that the cone 2 could be metallic as explained in the documents cited earlier and that it could also be added on the substrate 2.
An electrically insulating layer 9 is present on the surface 10 of the substrate 2. The insulating layer 9 carries a layer 11 of electrically conductive material and has an opening opposite to the well 3 so as to surround this latter. The layer 11 thus constitutes about the longitudinal axis 5 an annular electrode which is intended for example to constitute an electrode having a function corresponding to the function performed by a Wehnelt electrode as employed in particular in electron guns of cathode-ray tubes. Above the Wehnelt electrode 11 is deposited an electrically insulating layer 12 which is open opposite to the well 3 and separates the Wehnelt electrode 11 from a second electrically conductive layer 13. This second electrically conductive layer 13 is also open opposite to the well 3 so as to form a second annular electrode 13 which is centered about the longitudinal axis 5.
The second electrode 13 is brought to a positive potential of 100 volts, for example, with respect to a reference potential applied to the substrate 2, the Wehnelt electrode 11 being, for example, at a potential in the vicinity of or equal to that of the substrate 2.
Under these conditions, the point 7 emits electrons under the influence of the electric field produced by the potential of the second electrode 13 which thus constitutes an extracting electrode. The electrons emitted by the point 7 form an electron beam 6 which could if necessary be accelerated to a greater extent by means of additional electrodes. However, the electrode 13 could also be replaced by an anode without opening for the passage of the beam, as described in the French patent Application No. 2,568,394 which has already been cited.
The dimensions of the structure of the emitter device 1 are of the order of a few micrometers. By way of example, the diameter D of the well 3 can be two or three micrometers, the height H of the cone 4 can be of the order of one micrometer, and the radius of curvature (not shown) of the point 7 which constitutes the emissive point can be of the order of 0.06 micrometer. With this type of emitter device, it is possible to obtain an electron current, the mean intensity of which can be of the order of 25 microamperes and which can attain and even exceed (at a peak value) 100 microamperes.
A large number of these emitter devices can be associated in parallel and especially in the form of a matrix, thus obtaining the equivalent of an electron source or macroscopic cathode, the current of which can be modulated by the voltage of the extracting electrodes 13.
It is also possible to employ each elementary emitter device as a microtube electron gun, and to associate and combine a large number of such devices in order to form the equivalent of an integrated circuit, the semiconductor components being thus replace by vacuum microtubes.
Sources of this type offer many advantages. Compared with the cathodes and electron guns in conventional use, and especially in microwave tubes, they provide in particular the following advantages:
absence of heating and instantaneous operation; PA1 the possibility of modulating the current with a low modulation voltage and at low impedance, hence the possibility of operation with a very broad band; PA1 global current density considerably higher than the value which can be obtained in the present state of knowledge by making use of conventional means (at the present time, the maximum value is of the order of 10 amperes/cm.sup.2). PA1 the possibility of distinctly higher power per element; PA1 absence of losses within the material; PA1 distinctly higher microwave frequency efficiency; PA1 insensitivity to ionizing radiation; PA1 much greater immunity to electromagnetic pulses; PA1 possibilities of applications to visual display. PA1 considerable variation of emission from one emissive point to another as a function of the radius of curvature which cannot be controlled in practice; PA1 non-linearity of the modulation characteristic; PA1 substantial random variation in time of the current emitted by a point, this being due to the temporary presence on the point of residual gas molecules which modify the work function. It may even happen that the work function is reduced to such a point that the intensity of the current emitted by the point is sufficient to melt this latter by Joule effect. Furthermore, random current variation results in considerable noise; PA1 the electrons emitted by the point constitute a highly divergent beam which is practically non-refocusable.
With respect to semiconductor components, the advantages are as follows:
In spite of these numerous advantages, this technique is little used by reason of the fact that it is subject in particular to the following disadvantages: